Modern test and measurement instruments, such as portable oscilloscopes, generally employ LCD (Liquid Crystal Display) screens for displaying acquired waveforms and accompanying text. Some oscilloscopes, such as the TDS 200 oscilloscope manufactured by Tektronix, Inc. of Wilsonville, Oreg., employ an LCD of the type in which the waveform is displayed in one of several colors on a substantially white background. While this arrangement works quite well, some users prefer the waveform to be displayed on a black background (similar to the familiar cathode ray tube (CRT) display). If such a dark background display were to be output to a printer, the amount of ink used by the printer would be unacceptably high. At first, one might think that the solution to this problem is merely to invert the output before sending the file to the printer. This solution would be fine if only text were to be printed. Similarly, some graphics files might be inverted without causing an adverse result, but this is not true for the data representing the screen display of an oscilloscope, as will be explained below.
An oscilloscope displays a waveform as a function of amplitude with respect to time. Such a simplistic version of an oscilloscope display might be capable of color inversion before printing without the loss of information. However, the oscilloscope display also varies in intensity in accordance with how often a trace occurs at the same point on the screen. An oscilloscope having an LCD display operating in this manner is said to be mimicking the decay of illuminated phosphors of a CRT oscilloscope.
Another problem arises because modern oscilloscopes also display each channel in a different color, rather than only in white on black. Thus, the saturation of the particular color serves to indicate how often a particular point of the display has been illuminated within a given time. Selecting dark background colors for the waveform graticule and surrounding menu areas improves waveform display contrast and color separation, but also significantly increase hardcopy cost and printing time.
What is needed is an apparatus and method for inverting the colors of the display for printing, while maintaining the apparent relative intensity information of the display.